Many distillate hydrocarbon streams produced from crude oil contain some amount of sulfur in one form or another unless they have been subjected to sulfur removal procedures, such as hydrotreating. Often, a major amount of this sulfur is present in the form of mercaptan (thiol) compounds. It is normally desirable to remove at least some portion of the mercaptan sulfur from the hydrocarbon distillate stream in order to meet certain product specifications, such as a limitation on the total sulfur content of a hydrocarbon product. It may also be desirable to remove mercaptan compounds from a hydrocarbon stream for the purpose of eliminating the rather malodorous mercaptan compounds and thereby improve or reduce the odor associated with the hydrocarbon stream. A further reason for removing mercaptan compounds from a hydrocarbon stream is to eliminate the passage of sulfur-containing compounds into a catalyst bed that is sensitive to the presence of sulfur. For example, it may be desirable to remove mercaptans from a hydrocarbon distillate stream, such as a butane or gasoline type stream or a petrochemical feed stream, for the purpose of preserving the activity of a catalyst employed in a downstream conversion unit.
Mercaptan compounds are commonly removed from hydrocarbon streams through the use of an extraction process in which the hydrocarbon stream is brought into contact with an aqueous alkaline solution. The hydrocarbon feed stream is passed through an extraction column countercurrent to a descending stream of aqueous alkaline solution normally referred to in the art as “caustic.” The mercaptan compounds are preferentially dissolved in the aqueous alkaline “caustic” solution and are thereby extracted from the hydrocarbon stream. The mercaptan-containing caustic solution, which is referred to as a “rich” caustic solution, is then subjected to a procedure referred to as regeneration, which includes oxidizing the mercaptan compounds to disulfide (RSSR′) compounds. An oxidation catalyst dissolved in the caustic solution promotes the oxidation of the extracted mercaptan compounds to disulfide compounds. The process then continues with separating the disulfide compounds from the aqueous solution by decantation (or other physical separation means) in a phase separation vessel. The resultant “lean” caustic is removed from the separation vessel and recycled to the extraction column.
It is known to the those skilled in the art that a good separation of the disulfide compounds from the caustic solution is required in order to minimize the content of disulfide compounds in the caustic being recirculated to the extraction column. The disulfide compounds are often soluble in hydrocarbon streams. Therefore, disulfide compounds present in the regenerated caustic being fed to the top of extraction column will become dissolved in the hydrocarbon stream that is being treated. This will raise the sulfur content of the treated hydrocarbon stream beyond an acceptable level. It is known in the art to counteract this effect by removing disulfide compounds from the regenerated caustic. The regenerated caustic may therefore be processed by contacting with a hydrocarbon stream, such as naphtha, pentane, or hexane, to remove or “wash” the disulfide compounds therefrom. The “washed” caustic solution is then passed into the extraction column. It is also known in the art to pass the washed hydrocarbon stream into the separation vessel from which the disulfide oil phase is removed rather than contacting the aqueous solution withdrawn from this vessel in a separate step. Washing the regenerated caustic, however, increases the cost of hydrocarbon processing systems in terms of capital (equipment) costs, material costs, and energy costs.
Accordingly, it is desirable to provide improved methods for treating hydrocarbon streams containing mercaptan compounds. Additionally, it is desirable to provide such methods that are able to remove disulfide compounds from the regenerated caustic without the need for separate washing apparatus/procedures. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.